Modern seat belt restraining systems utilize a three-point belt that is anchored on the B-pillar above the occupants shoulder, to the floor next to the B-pillar, and to an inboard buckle positioned next to the seat in a region adjacent to the occupant's hips. The inboard buckle secures two portions of the seat belt strap to the inboard anchor point, the portion that extends from the B-pillar over the occupant's outboard shoulder across the chest and to the waist on the inboard side, and the portion that extends from the outboard anchor point on the floor to the waist of the occupant.
A two-way seat is one that moves fore and aft, with no up and down position adjustment of the seat possible. An inboard buckle may be attached to a two-way seat without comprising the safety provided by the seat belt since the seat can be securely fastened to the upper rail and the engagement of the upper rail with the lower rail is one which will resist occupant loads on the belt in a crash event.
A four-way seat is one that is adjustable fore and aft, and also up and down to accommodate the physical dimensions and comfort of the occupant. Not only does a four-way seat provide additional motion to the seat that has to be accommodated in the design of the seat belt and the inboard buckle, it also introduces another element into the support system for the seat, pairs of levers that are pivoted together to open and close with a folding motion to raise and lower the seat. The pivoted levers are prone to distortion in a crash event and hence result in seat motion. Further, because of the location and design of the pivoted levers, in a crash event, the levers tend to unfold, pitching the seat forward and upward. If the inboard buckle is attached to the seat, this motion of the seat creates slack in the seat belt.